The two-dimensional mathematical model for water exchange in Maowei sea is established using the dissolved conservative material as the tracer. The pollutions are fed back at boundary under assumption that the gradient of pollutant concentration is the same both inside and outside the boundary.Based on the validation of the mathematical model,the effect on the volume of tidal prism and water change of Maowei sea is calculated and analyzed by the reclamation of the Qinzhou bay mouth.Results indicate that the width of Longmen strait is the key factor for keeping the volume of tidal prism of Maowei sea.The reclamation engineering in both sides of Qinzhou bay mouth keep the width over 9.4 km and has little effect on the water half-exchange period and the tidal prism of the Maowei sea.